1. Field of the Invention
The present invention relates to a MOS (metal oxide semiconductor) memory device. More particularly, it relates specifically to a so-called EPROM (electrically programmable read-only-memory), EEPROM (electrically erasable programmable read only memory), or flash memory, i.e., to a type of semiconductor memory comprising a floating gate and a control gate, in which the information is stored by the carriers that are captured in the floating gate upon applying high voltage to the control gate.
In contrast to the conventional memory devices in which the repetition of erasing and writing of information is limited to a predetermined value due to damage of gate insulator films and the like at the introduction of carriers, the present invention provides, particularly, a semiconductor device which can be subjected far more frequently to erasing and writing. The application field of the memory devices, which was confined to a certain field for the conventional devices, is now extended not only to novel fields but also to the field in which other types of memory are now utilized. Accordingly, the present invention is applicable not only to the fields in which conventional EEPROM, EPROM, etc., are used to the present, but also to various fields in which RAMs (random access memories) are used.
2. Prior Art
Many research and development efforts are now paid for realizing finer, more highly integrated, and more functional semiconductor devices. In particular, the most remarkable progress can be found in the insulated gate field-effect semiconductor devices called MOSFETs (metal oxide semiconductor field-effect transistors). The metals used herein not only refer to pure metals, but it collectively encompasses semiconductor materials having sufficiently high electric conductivity, alloys of semiconductors with metals, and the like. The oxide to be placed between the metal and the semiconductor not necessary be a pure oxide, and insulators such as nitrides are sometimes used. In this context, the term "MOS" is not applicable to such structures in a strict sense; in the descriptions hereinafter, however, devices of field effect type having the structure above, inclusive of those comprising nitrides or other insulators in the place of oxides, are referred to as MOSFETs or MOS transistors.
A typical MOS transistor comprises a semiconductor substrate having provided thereon an oxide (insulator) such as silicon oxide as a gate oxide (gate insulator), and further thereon a metal, a semiconductor, etc., which functions as a gate electrode. The conductivity of the base semiconductor is controlled by varying the voltage which is applied to the gate electrode.
Also, there is known that a nonvolatile memory can be fabricated by establishing an electrically independent semiconductor film (i.e., a floating gate) on the gate oxide, and then providing thereon a gate electrode (i.e., a control gate). Those memories are commercially available as EPROMs and EEPROMs. In the memories of this type, the conductivity of the base semiconductor is semi-permanently fixed by applying an intense electric field to the gate electrode, thereby trapping charges such as electrons and holes in the floating gate film. Thus, of course, if the charge having implanted into the floating gate is removed by using an ultraviolet (UV) irradiation or an electric effect, the initial state can be recovered and hence the stored data can be erased. A semiconductor memory device can be assembled from such floating gate type M0S transistors alone, or from a combination of those with other transistors.
The memories using the elements described above are, in contrast to the RAM devices such as DRAMs (dynamic random access memory) and SRAMs(static random access memory), operable without using a power source. Furthermore, when compared with a DRAM, in particular, the memories of the type above can be used free of capacitors. Hence, they are now the subject of active study because they can minimize the cell area per bit in fabricating a memory of 16 MB or more in size and are thereby best suited for realizing higher integration. Among them, particular attention is paid to the EEPROMs.
It should be noted, however, that the MOSFETs having the floating gate above are different from the RAMs in one point, i.e., that the MOSFETs of this type cannot be subjected to writing and erasing processes for an infinite times. Thus, ROM devices are put into one category separately from the RAMs. It is guaranteed for a commercially available EEPROM to endure writing and erasing data for 10,000 to 100,000 times. If such a memory device were to be assembled into a computer as a temporal data memory in the place of a conventionally employed RAM, it must be guaranteed to endure for at least 100,000 times, and preferably 1,000,000 times of repeated writing and erasing processes.
The conventional types of EEPROMs can resist only to limited times of writing and erasing, because the gate oxide film under the floating gate suffers serious damage upon implanting carriers into or drawing them out from the floating gate. More specifically, the interatomic bonds inside the insulator layer (a silicon oxide film, in general) are cut by the high energy carriers passing therethrough to form defects such as trap levels. Once such defects are formed, the carriers easily move through the insulator layer and allow the charge having accumulated in the floating Mate to escape. If such a phenomenon occurs, the EEPROM no longer functions as a memory device, or its reliability is almost lost.
It is preferred to use a material having a strong interatomic bond for the insulator layer to cope with the aforementioned problems. For example, silicon nitride and aluminum oxide can be suitably used as the insulator layer. However, those insulating materials, on the other hand, develop poor interfacial states with a semiconductor (silicon) substrate; i.e., the development of a trap level disturbs electric conductivity in the channel region of the MOS or lowers the mobility of the carriers.
The EEPROM is believed promising for use as a flash memory, i.e., a special type of a nonvolatile memory to replace DRAMs, but the market is yet to be matured due to the limits concerning the repetition of writing.